Response based design conditions in the North Sea: Application of a new method

The estimation of rare, extreme environments from a relatively short oceanographic database is crucial to the calculation of loads and responses of offshore structures. A novel and consistent approach has been developed that makes use of a number of asymptotic properties of extremes. A storm can be adequately characterized by its most probable extreme wave or the resultant structural response. This allows them to treat storms, rather than sea states, as the essential random, independent events; to account correctly for uncertainty in the largest wave and structural response within a storm; and to deal with the uncertainty in the severity of randomly arriving storms, including the rare storms that are more severe than those in the database. The method provides the long term load statistics essential to reliability analysis and to the calibration of environmental load factors in design codes. The demonstrate the application of the method to the prediction of extreme waves, loads and ``response based`` environmental design conditions for a drag dominated structure in the North Sea.